Digital terrestrial broadcast in Japan began in 2003, and a service of digital terrestrial broadcast began in Europe and other districts from 2000. Meanwhile, as portable terminals become popular, there have been provided various high value added services such as a mail function in comparison with a telephone function of a conventional portable terminal. In such a trend, portable terminal manufacturers in the world consider to bring in a service of receiving digital terrestrial broadcast by giving a portable terminal a reception function of digital terrestrial TV broadcast in addition to a conventional portable telephone function thereby to make a portable terminal with a digital terrestrial TV receiver.
A digital terrestrial TV tuner module for a portable terminal as above, being aimed for a portable terminal, is required to be less power-consuming, smaller and shorter, in addition to coping with a problem in terms of a characteristic of a tuner in itself such as reception sensitivity, and is further required to prevent a transmission wave from blocking a reception wave.
FIG. 31 shows a constitution diagram of a transmission/reception section of a portable terminal having a digital terrestrial TV receiver. In a portable terminal capable of receiving digital TV broadcast, due to a difference between frequency bands of audio (data) communication and TV broadcast, two antennas of an antenna 1 for receiving digital TV broadcast and an antenna 2 for transmitting/receiving an audio (and data) communication signal are closely disposed. Here, usually, a digital TV broadcast wave is a weak radio wave (−90 dBm), and thus reception sensitivity of a TV tuner module 10 is designed to be quite high. On the other hand, a transmission wave of an audio transmission section 20 transmitting audio (and data) as the portable terminal makes the antenna 2 emit a very strong radio wave (about +30 dBm). Therefore, the transmission wave of audio (and data) communication reaches the TV broadcast tuner module 10 via the antenna 1 and interferes reception of TV broadcast. Thus, it is said that a D/U ratio (Desired/Undesired Ratio: power ratio between desired wave and ghost wave) of 120 dB is required for a portable terminal having a digital terrestrial TV receiver.
Incidentally, a reception band of digital TV broadcast in Japan is 470 MHz to 770 MHz and a transmission band of audio communication exists closely thereto in a higher frequency side than the reception band. The reception bands are different by communication common carriers and at present there exist transmission bands of 824 to 830 MHz, 898 to 925 MHz, and 1940 to 1960 MHz. FIG. 32 is a characteristic chart showing a pass band and attenuation bands each corresponding to the reception band of digital TV broadcast and transmission bands of audio communication used by the communication common carriers.
As already described above, the antennas are closely disposed and in a case that bands of transmission/reception waves are close to each other, there occurs a problem that a transmission wave of audio or data communication reaches the digital TV broadcast reception antenna via the antenna thereby giving interference to a weak reception wave of TV broadcast. In order to avoid this interference, it is required to provide a filter having a steep attenuation amount at a root of the reception antenna for digital TV broadcast.
FIG. 33 shows a constitution example of a reception system of a digital terrestrial TV tuner module which satisfies the above-described requirement. A BEF circuit (band elimination filter circuit) 11 has a steep suppression (−50 to −60 dB) characteristic for a transmission wave frequency band discharged from a transmission/reception antenna (not shown) of a cellular phone, and takes in a digital terrestrial TV wave received by an antenna 1 at a low loss. An LC filter 12, constituted by a chip coil and a chip inductor, suppresses (−20 to −40 dB) the transmission wave frequency band. A balun circuit 13, while suppressing (−10 to −20 dB) the transmission wave frequency band, performs balance-unbalance conversion on a TV broadcast wave. An IC circuit 14 converts a TV signal to become a modulated wave into a base band.
A characteristic required for a BEF circuit is having a low loss in a reception band of digital TV broadcast (small attenuation amount), and having a steep attenuation and a large attenuation amount in a transmission band of audio or data broadcast. As a BEF circuit, there is generally a passive circuit using a coil or a capacitor, a dielectric filter utilizing a high Q value of a dielectric, and a laminated chip component such as LTCC and HTCC in which the passive circuit is disposed on a pattern and further sintered. However, a module mounted in a portable terminal is restricted in terms of a mounting area and a space, and the above constitution has its limit. Further, since the reception band of digital TV broadcast and the transmission band of audio or data are quite close to each other, a steep attenuation characteristic is necessary. In a conventional filter design theory, in order to is raise steepness, resonance circuits being constituents of a filter are connected in two stages or more, so that a loss in a pass band becomes quite large.
Then, as a filter enabling small power consumption, miniaturization and shortness, there is an SAW (surface acoustic wave) filter. However, it is difficult to obtain a broad pass band and a large attenuation amount by constituting the SAW filter by an SAW resonator alone. On the other hand, Patent Document 1 describes a constitution in which SAW resonators being parallel arms are connected in a plurality of stages via an inductor provided in a transmission path (signal path), and if this method is adopted, it is possible to obtain a pass band with a small attenuation amount and an attenuation band which is close to this pass band and in which a large attenuation amount can be obtained. FIG. 34 shows an SAW filter constituted by connecting SAW resonators 31 being seven parallel arms and inductors for phase inversion (for displacing a phase by 90 degrees), with a reference numeral 33 indicating an input port and a reference numeral 34 indicating an output port.
When an attenuation band from 824 to 830 MHz is called a first attenuation band and an attenuation band from 898 to 925 MHz is called a second attenuation band, SAW resonators resonating in parallel in the first attenuation band are allotted to three of seven SAW resonators 31 and SAW resonators resonating in parallel in the second attenuation band are allotted to the remaining four SAW resonators. FIG. 35 shows a filter characteristic of this circuit, and it can be known that an excellent filter characteristic fulfilling the above requirement can be obtained. In this SAW filter, a still large attenuation amount can be secured by increasing the number of connection stages of the SAW resonators 31.
Incidentally, since the number of inductors increases as the number of connection stages of the SAW resonators, a size of a device becomes large due to the following reason. When the inductor is to be created according to a wiring pattern, it is necessary to bend a large pattern and to make a film thickness large, making a conductor loss large and lowering a Q value. Thus, it is practical to use an external coil. FIG. 36 is a schematic perspective view of an SAW filter in a case that an external coil is used, and a reference numeral 35 indicates a wiring board, a reference numeral 36 indicates an SAW device in which SAW resonators are formed, and a reference numeral 37 indicates the external coil. As can be known from this diagram, when the mount number of the external coils is large the size of the device becomes large, which is a large cause for hampering miniaturization of the device.
Since a portable terminal is required to be still smaller due to increased variation of functions and the like, a technology capable of further downsizing an SAW filter device is requested.
Patent Document 2 describes a technology to connect a plurality of SAW resonators having different resonance frequencies from each other to the same electric potential point and to combine these plural resonance frequencies thereby to constitute one band-pass filter, but does not describe a filter to cope with steep attenuation bands close to each other.